Method and system for calibration of a fingerprint sensing device

ABSTRACT

There is provided a method of initiating a calibration operation of a fingerprint sensing device comprising an array of sensing elements for sensing a fingerprint pattern, the method comprising acquiring information indicative of a change of an environmental parameter influencing the operation of the fingerprint sensing device; and if the change is larger than a predetermined threshold value, performing a calibration operation of the fingerprint sensing device. There is also provided a method of initiating a calibration operation of a fingerprint sensing device comprising an array of sensing elements for sensing a fingerprint pattern, the method comprising: receiving information indicative of an event in a device in which the fingerprint sensing device is arranged; if the event is one of a group of predetermined events, performing a calibration operation of the fingerprint sensing device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Swedish Patent Application No.1551471-4 filed Nov. 13, 2015. The disclosure of the above applicationis incorporated herein by reference in its entirety.

FIELD

The present invention relates to calibration of a fingerprint sensor. Inparticular, the present invention relates to a method for determiningwhen to initiate a calibration of a fingerprint sensor comprised in anelectronic device.

BACKGROUND

Various types of biometric systems are used more and more in order toprovide for increased security and/or enhanced user convenience. Inparticular, fingerprint sensing systems have been adopted in, forexample, consumer electronic devices, thanks to their small form factor,high performance, and user acceptance.

Among the various available fingerprint sensing principles (such ascapacitive, optical, thermal etc.), capacitive sensing is most commonlyused, in particular in applications where size and power consumption areimportant issues.

All capacitive fingerprint sensors provide a measure indicative of thecapacitance between each of several sensing structures and a fingerplaced on or moved across the surface of the fingerprint sensor.

Electrical properties (determining analogue signal strengths) of afingerprint touch sensor depend on external factors. In particular thepresence of moist between the finger and the sensor can make asignificant difference. Moist can come from the user's finger or bepresent on the sensor already before the finger is presented. Moist onthe finger is typically due to sweating or recent contact with water.Moist on the sensor can also be due to direct contact with water, e.g.after the user has taken a bath or when the device is used in rainyweather, or due to condensation of water from humid air at the sensorsurface. In the latter case the amount of condensed water is furtherdependent on air pressure, temperature, and temperature differencebetween the air and the sensor surface.

Electromagnetic fields may also affect the electrical properties of thesensor. An electromagnetic field interacting with the sensor circuitrymay induce power in circuits and connections, resulting in a shift inthe change in electrical potential required to reach the analogue fingerdetect threshold. This may lead to that the fingerprint image iscaptured too early or too late.

In order to compensate for analog signal strength variations dependingon external factors, trigger thresholds and parameters foranalogue-to-digital conversion can be calibrated towards the currentconditions. This can be done automatically by various calibrationmethods, but typically requires that no finger is placed on the sensorduring the calibration. Furthermore, a calibration is often performed atsystem startup and thereafter repeated with given time intervals.

However, continuous calibration at given time intervals may lead to anunnecessarily high power consumption when the fingerprint sensor isunused for long periods of time. The power consumption may be decreasedby increasing the time between calibrations. However, this is anundesirable approach since increasing the time between calibrations maylead to situations where the sensor is being used just prior to ascheduled calibration event, increasing the risk that the fingerprintsensor is not properly calibrated for the current operating conditions.

Accordingly, there is a need for an improved method of calibrating afingerprint sensor.

SUMMARY

In view of above-mentioned desired properties of a fingerprint sensor,it is an object of the present invention to provide a method fordetermining when to perform a calibration operation ensuring a moreaccurate and power efficient operation of a fingerprint sensor.

According to a first aspect of the invention, there is provided a methodof initiating a calibration operation of a fingerprint sensing devicecomprising an array of sensing elements for sensing a fingerprintpattern, the method comprising acquiring information indicative of achange of an environmental parameter influencing the operation of thefingerprint sensing device; and if the change is larger than apredetermined threshold value, performing a calibration operation of thefingerprint sensing device.

The fingerprint sensing device, i.e. the fingerprint sensor, may forexample be a capacitive fingerprint sensor where a fingerprint image isacquired by measuring a capacitive coupling between a finger placed onthe sensor and the sensing element, thereby determining the distancebetween the sensing element and the finger, such that ridges and valleysof the finger are identified. A fingerprint sensor arranged in anelectronic device is typically calibrated at selected points in time tosetup the fingerprint sensor for image capture under the currentconditions.

The present invention is based on the realization that the calibrationof the fingerprint sensor can be triggered to occur only when it isrequired, e.g. only when the external environment of the fingerprintsensor has changed to such a degree that it is unlikely that the latestperformed calibration would provide an optimal, or satisfactory,fingerprint image. Accordingly, it is suggested to only initiate andperform a calibration of the fingerprint sensor only when an acquiredparameter value exceeds a predetermined threshold value. The calibrationfunctionality in itself can be assumed to be a known feature of thefingerprint sensing device. The present method is thus aimed atcontrolling when a calibration operation is initiated, i.e. when thecommand is given to perform one or more specific calibration operations.The calibration operations may comprise various known measures relatingto the functionality of the sensor, and the specific calibrationoperations to perform will not be discussed in great detail.

Through the inventive method, the power efficiency of a fingerprintsensor can be improved since it is no longer required to performcalibrations at regular time intervals for the sensor to be capable ofcapturing a fingerprint image. Moreover, the reliability of the sensoris improved since the sensor is more likely to be calibrated accordingto current conditions when a fingerprint image is to be captured.

According to one embodiment of the invention, the step of acquiringinformation may comprise determining a present value of an environmentalparameter, determining a difference between the present value of theenvironmental parameter and a predetermined value of the environmentalparameter, and if the difference is larger than the predeterminedthreshold value, performing the calibration operation of the fingerprintsensor. Determining a present value may comprise controlling a sensorconnected to the fingerprint sensor to measure a parameter value, and tocompare the current value with a predetermined value.

In one embodiment of the invention, the predetermined value of theenvironmental parameter may be a value determined at the last performedcalibration operation. The predetermined value may also be an averagevalue based on a number of earlier values. The predetermined value mayalso represent one or more predetermined parameter value ranges, suchthat when the current value is within a particular predetermined range,a calibration of the fingerprint sensor is performed.

According to one embodiment of the invention, the environmentalparameter may be selected from the group comprising humidity,temperature and electrical field strength, all of which may influencethe operation of a fingerprint sensor. Taking the example of acapacitive fingerprint sensor, the electromagnetic field surrounding thesensor can influence the electrical potential of conductive portions ofthe sensor, or of the finger, which in turn may influence thefingerprint capture. Furthermore, moist or humidity may influence thedielectric environment between the finger and a sensing element of thesensor, in turn influencing the capacitive coupling. The temperature mayalso influence the fingerprint capture, not in the least indirectlysince a higher temperature leads to a higher risk that the finger ismoist due to sweating. A rapid change in temperature may also affect thecondensation at the surface of the fingerprint sensor.

According to one embodiment of the invention, determining a presentvalue of an environmental parameter may comprises acquiring informationfrom a sensor integrated in the fingerprint sensor. The fingerprintsensor may comprise dedicated sensors for determining an environmentalparameter. It may also be possible to determine a value of anenvironmental parameter, such as electric field, by using componentsalready present in the fingerprint sensor.

According to one embodiment of the invention, determining a presentvalue of an environmental parameter may comprise acquiring informationfrom a sensor external to the fingerprint sensor. The fingerprint sensordevice is typically integrated in a consumer electronics device, whichin itself may comprise one or more sensors for determining parameterssuch as temperature, humidity and electromagnetic field. Thereby, thefingerprint sensor may request information for such external sensors.

Furthermore, determining a present value of an environmental parametermay comprises acquiring information via a network connection. Thefingerprint sensor device may be integrated in a device having a networkconnection, such as a WiFi, Bluetooth or cellular data connection. Thismeans that the device may communication with remote devices or withinternet services to acquire relevant environmental information. To berelevant, the acquired information is preferably based on the locationof the device, and the location may be determined using a GPS integratedin the device or by means of location identification via the networkconnection.

According to one embodiment of the invention, the method may furthercomprise performing a second calibration operation of the fingerprintsensor a predetermined time after a first operation has been performedresulting from the change in environmental parameter. It may bedesirable to perform a second calibration after a certain time, when thesensor has fingerprint sensor has reached has stabilized after thedetected change. For example, if a temperature change is detected,triggering a calibration operation, it may be some delay before allcomponents of the sensor has reached the new temperature. Therefore, itis advantageous to perform a second calibration at a time when it isestimated that a stable state of the sensor is reached.

According to a second aspect of the invention, there is provided amethod of initiating a calibration operation of a fingerprint sensordevice comprising an array of sensing elements for sensing a fingerprintpattern, the method comprising: receiving information indicative of anevent in a user device in which the fingerprint sensor device isarranged; if the event is one of a group of predetermined events,performing a calibration operation of the fingerprint sensing device.

The second aspect of the invention is based on the realization that itmay be sufficient to calibrate the fingerprint sensor when an event isdetected that anticipates the use of the fingerprint sensor. Taking theexample of a fingerprint sensor arranged in a user device such as asmartphone, there are typically long periods of inactivity of thesmartphone when there is no need for the fingerprint sensor to berecently calibrated, and where regular calibrations would only lead toincreased power consumption. Instead, the occurrence of a specific eventmay be used to initiate a calibration operation, thereby limiting thecalibrations to when there is a high likelihood that the fingerprintsensor is about to be used. Information indicative of an event is thusreceived by the fingerprint sensing device, or by a control unitconfigured to control calibration of the fingerprint sensing device.

According to one embodiment of the invention, the group of predeterminedevents may comprise events requiring a user interaction with the devicein which the fingerprint sensor is arranged. Such events may for examplecomprise an incoming call, an alarm, an incoming message and an incomingnotification, where the user is prompted to use the device. Thereby,when such an event is detected, a calibration operation can be initiatedand performed before the user has time to reach the sensor since acalibration operation can be performed in a very short time compared tothe time it takes for a user to react to an event. Thus, the fingerprintsensor can be calibrated just prior to being used. To perform therequired calibration operation may take in the range 1-2 seconds.However, the time for performing the calibration operating may varydepending on the hardware used on the type of calibration to beperformed. Moreover, it may be desirable to perform different types ofcalibration operations for different types of events, or for differentchanges in an environmental parameter.

According to one embodiment of the invention, the event may comprise achange in a network connection. The change in network connection may forexample be a change in cell ID in a cellular network, a change in WiFinode connection, a change in a Bluetooth connection or the like.Furthermore, the event may comprise a change in location. The change inlocation may be detected for example as a change in network connection,where the change from one WiFi node to another can be used as anindicator of a change in location. Changing location may in turnindicate that the external environment changes, such as if a user ismoving from indoors to outdoors, which may require a calibration of thefingerprint sensor.

According to one embodiment of the invention, the event may comprise theconnection of a charger to the device in which the fingerprint sensingdevice is arranged. When a charger is connected to the device, thesupply voltage may change, and in particular the drive voltage V_(DD) ofthe fingerprint sensor, which in turn influences the operation of thesensor. Thereby, it may be desirable to perform a calibration when acharger is connecter. Furthermore, a charger may introduce noise intothe device, and into the fingerprint sensor, such that a calibration isrequired to properly capture a fingerprint despite any induced noise.

According to on embodiment of the invention, the method may furthercomprise waiting for a minimum period of time between consecutivecalibration operations. There may be situations where a device receivesa large number of notifications, or when other types of events occur,during a short period time, in which case it is not desirable that acalibration operation of the sensor is performed after every event.Thereby, a minimum time between calibrations can be set to both toensure that the sensor is not occupied with calibration operations whenit is to be used, and to reduce the power consumption of the sensor.

Additional effects and features of the second aspect of the inventionare largely analogous to those described above in connection with thefirst aspect of the invention.

According to a third aspect of the invention, there is provided afingerprint sensing device comprising an array of sensing elements forsensing a fingerprint pattern, said device comprising a calibrationcontrol unit configured to acquire information indicative of a change ofan environmental parameter influencing the operating of said fingerprintsensor, if said change is larger than a predetermined threshold value,perform a calibration operation of said fingerprint sensor, receiveinformation indicative of an event in a device in which said fingerprintsensor device is arranged; and if the event is one of a group ofpredetermined events, perform a calibration operation of the fingerprintsensor. The fingerprint sensing device may for example be a capacitivesensing device.

Furthermore, the fingerprint sensing device may comprise a temperaturesensor, a humidity sensor, and/or an electromagnetic field sensor. Thefingerprint sensing device may also be arranged in a user device, suchas a smartphone, where the above described sensors are integrated in thesmartphone and communicatively coupled to the calibration control unitof the fingerprint sensing device.

Effects and features of the third aspect of the invention are largelyanalogous to those described above in connection with the first andsecond aspects of the invention.

Further features of, and advantages with, the present invention willbecome apparent when studying the appended claims and the followingdescription. The skilled person realize that different features of thepresent invention may be combined to create embodiments other than thosedescribed in the following, without departing from the scope of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing anexample embodiment of the invention, wherein:

FIG. 1 schematically illustrates a smartphone comprising a fingerprintsensing device;

FIGS. 2A-B are flow charts outlining the general steps of a methodaccording of an embodiment of the invention;

FIG. 3 is a schematic illustration of a device comprising a fingerprintsensing device according to an embodiment of the invention;

FIG. 4 is a flow chart outlining the general steps of a method accordingof an embodiment of the invention;

FIG. 5 is a schematic illustration of a device comprising a fingerprintsensing device according to an embodiment of the invention;

FIG. 6 is a schematic illustration of a device comprising a fingerprintsensing device according to an embodiment of the invention;

FIG. 7 is a schematic illustration of a system and a device comprising afingerprint sensing device according to an embodiment of the invention;and

FIGS. 8A-B are timelines schematically illustrating the method accordingto embodiments of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the present detailed description, various embodiments of the methodand device according to embodiments of the present invention are mainlydescribed with reference to a capacitive fingerprint sensing devicearranged in a handheld user device, such as a smartphone. However, themethod is applicable also for other types of fingerprint sensingdevices, and for fingerprint sensing device arranged in other types ofconsumer electronic devices.

FIG. 1 schematically illustrates an application for a fingerprintsensing device 102 according to an example embodiment of the presentinvention, in the form of a smartphone 100 with an integratedfingerprint sensing device 102. The fingerprint sensing device 102 may,for example, be used for unlocking the smartphone 100 and/or forauthorizing transactions carried out using the smartphone 100, etc. Afingerprint sensing device 102 according to various embodiments of theinvention may also be used in other devices, such as tablet computers,laptops, smartcards or other types of consumer electronics.

FIG. 2A is a flow chart outlining the general steps of a methodaccording to an embodiment of the invention, with reference to the blockdiagram of FIG. 3 illustrating features of a fingerprint sensing deviceand of a smartphone in which the sensing device is arranged. First, themethod comprises acquiring 202 information indicative of a change of anenvironmental parameter influencing the operation of the fingerprintsensor. The acquisition of information can be initiated by a requestfrom a calibration control unit 302 of the fingerprint sensing device102. Alternatively, or in combination, information indicative of achange of an environmental parameter may also be provided to the controlunit 302 of the fingerprint sensing device 102 from the smartphone 100,without a specific request from the control unit 302. The control unit302 is illustrated herein in the form of a microprocessor, connected tothe fingerprint sensing device 102, the control unit 302 beingconfigured to control at least the initiation of a calibration operationof the fingerprint sensing device 102. The control unit 302 may also beconfigured to control all functionality of the fingerprint sensingdevice 102. Moreover, the control unit 302 may be a general purposecontrol unit of the smartphone 100, also configured to control thefingerprint sensing device 102.

Next, the change in parameter value is compared 204 to a predeterminedthreshold value, and if the change is larger than the threshold value, acalibration operation the fingerprint sensor is initiated 206 andsubsequently performed so that the fingerprint sensing device 102 iscalibrated according to the current environmental conditions.

As illustrated in the flow chart of FIG. 2B, determining a change in anenvironmental parameter may comprise determining 208 a present value ofan environmental parameter, determining 210 a difference between thepresent value of the environmental parameter and a predetermined valueof the environmental parameter; and comparing 212 the difference with apredetermined threshold value, and if the difference is larger than thepredetermined threshold value, initiating 214 and subsequentlyperforming the calibration operation of the fingerprint sensor.

Accordingly, as illustrated in FIGS. 2A-B, the information indicative ofchange may either be provided directly as information from a sourceexternal to the control unit 302 and fingerprint sensor 102, or thechange may be detected by a measurement and subsequent comparisoninitiated by the control unit 302 of the fingerprint sensor 102.

Parameters that may be calibrated during a calibration operation of thefingerprint sensor include, but are not limited to, a threshold foranalogue finger detection, analogue gain before analog-to-digital (AD)conversion of a pixel value, and digital gain and shift in AD-conversionof a pixel value. In the case where the AD-conversion parameters aretuned for each image capture, the default settings or start values fortuning can be subject to calibration.

FIG. 3 further illustrates that the control unit is communicativelyconnected to a number of sensors for measuring environmental parameters.The sensors are here illustrated as a temperature sensor 304, a humiditysensor 306, and an electromagnetic field sensor 308. The environmentalsensors may be integrated in the fingerprint sensing device circuitry,or they may be part of the smartphone 100 in which the fingerprintsensor 102 is arranged.

FIG. 4 is a flow chart outlining the general steps of a method accordingto an embodiment of the invention comprising receiving 402 informationindicative of an event in a device 100 in which the fingerprint sensingdevice 102 is arranged, and determining 404 if the event is one of agroup of predetermined events. If the event is among the group ofpredetermined events a calibration operation of the fingerprint sensoris initiated 406 and subsequently performed.

The group of predetermined events comprises events requiring a userinteraction with the smartphone 100. As illustrated in FIG. 5, the groupof events may comprise an incoming call 502, an alarm 504, an incomingmessage 506 and an incoming notification 508. All of the aforementionedevents typically encourage a user to interact with the smartphone 100,possibly requiring the use of the fingerprint sensor 102 for unlockingthe smartphone 100. Other types of events may also be comprised in thegroup of predetermined events. An example of such an event is theconnection of the smartphone 100 to a charger, which may lead to achange in the drive voltage, V_(DD), of the fingerprint sensing device,which in turn influences the operating characteristics of the device andthereby benefiting from a re-calibration of the fingerprint sensor.Other events triggering a calibration operation may be events relatingto applications of the device, for example if an application is launchedwhich can be anticipated to require fingerprint verification.

FIG. 6 schematically illustrates another group of events which may becomprised in the group of events initiating a calibration operation.FIG. 6 illustrates that the smartphone 100 in which the fingerprintsensor 102 and control unit 302 is arranged may comprise variousadditional functionality, such as WiFi 602, Bluetooth 604, Navigation606 and cellular network 608 connection functionality. The describedfunctionalities can all be used to indicate a change of location of thesmartphone 100, which in turn may require a calibration of thesmartphone 100. For example, a change in signal strength, or the loss ofa WiFi connection may indicate that the user moves from indoors tooutdoors, thereby suggesting a change in environmental conditionsrequiring a calibration operation. The same applies for a change in aBluetooth connection or the change in a cellular network connection.Furthermore, a device is often wirelessly connected via two or moretechnologies, in which case the combination of changes in connectionproperties can be used to determine for example if the user moves fromindoors to outdoors. Navigation functionality using systems such as GPS,Glonass or Galileo can also be used to determine a change of location ofthe smartphone 100 requiring a calibration of the fingerprint sensor102.

FIG. 7 illustrates a user device 700 comprising at least one of a WiFi602, Bluetooth 604 and a cellular network 608 connection and which isconfigured to connect to a remote service provider 702 to acquireinformation regarding an environmental parameter, a change inenvironmental parameter, and/or of an event which can be used todetermine if a calibration operation should be initiated. The remoteservice may for example be an internet based service providingup-to-date local humidity and temperature information. The remoteservice may also be connected to specific sensors 704 located in thevicinity of the user device 700, which may be of any of the typesdiscussed above. Moreover, the user device 700 may also use apeer-to-per connection to exchange relevant environmental informationwith nearby devices, thereby receiving relevant information indicativeof a change in an environmental parameter. Thereby, a change in anenvironmental parameter may be detected even if neither of thefingerprint sensing device 102 or the user device 700 in which thesensor is arranged comprises any sensors.

FIGS. 8A-B schematically illustrate timelines elaborating further stepsof a method according to embodiments of the invention. FIG. 8Aillustrates that a second calibration operation is performed apredetermined time t₁ after a first calibration operation has beenperformed, where the first calibration operation was initiated by achange in environmental parameter or by a specific event. If the changein environmental parameter triggering the first calibration is a rapidchange in temperature, it may be advantageous to perform a secondcalibration once the properties of the fingerprint sensing device havestabilized to the new conditions. Taking the example that a change intemperature is detected where the temperatures changes from +20° C. to−10° C., indicating a movement from indoors to outdoors, a firstcalibration is advantageously initiated and performed to compensate forany change in humidity. After a certain time has elapsed, such as one ora few minutes, it can be assumed that the temperature of the surface andof the components in the sensing device has stabilized after which asecond calibration is performed.

FIG. 8B illustrates the situation where a first calibration isinitiated, for example based on an event such as a message notification,and where a number of subsequent event occur in rapid succession, andwhere the events normally would trigger new calibrations. To avoidperforming many calibrations repeatedly in a short time interval, a timeperiod t₂, starting after the first calibration is finalized, is setduring which no calibration may be performed. It may also be possible tooverride the no calibration condition during t₂ is specific excludedevents occur, such as a change in the temperature. Once t₂ has passed,the next event triggering a calibration can be handled normally and acalibration performed.

It is also easy to realize that a combination of the above describedmethods for initiating a calibration operation of a fingerprint sensingdevice may be used to an advantage. For example, if a calibration hasbeen recently performed resulting from a change in an environmentalparameter, it may not be required to perform a new calibration if anevent from the group of events is detected.

Even though the invention has been described with reference to specificexemplifying embodiments thereof, many different alterations,modifications and the like will become apparent for those skilled in theart. Also, it should be noted that parts of the method may be omitted,interchanged or arranged in various ways, the method yet being able toperform the functionality of the present invention.

Additionally, variations to the disclosed embodiments can be understoodand effected by the skilled person in practicing the claimed invention,from a study of the drawings, the disclosure, and the appended claims.In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

1-18. (canceled)
 19. A fingerprint sensing device comprising an array ofsensing elements for sensing a fingerprint pattern, said devicecomprising a calibration control unit configured to: acquire informationindicative of a change of an environmental parameter influencing theoperating of said fingerprint sensing device; if said change is largerthan a predetermined threshold value, perform a calibration operation ofsaid fingerprint sensing device; receive information indicative of anevent in a device in which said fingerprint sensing device is arranged;and if said event is one of a group of predetermined events, perform acalibration operation of said fingerprint sensing device.
 20. Thefingerprint sensing device according to claim 19, further comprising atemperature sensor, a humidity sensor, and/or an electromagnetic fieldsensor.
 21. The device according to claim 19, wherein said control unitis further configured to: determine a present value of an environmentalparameter; determine a difference between said present value of saidenvironmental parameter and a predetermined value of said environmentalparameter; and if said difference is larger than said predeterminedthreshold value, perform said calibration operation of said fingerprintsensor.
 22. The device according to claim 21, wherein said predeterminedvalue of said environmental parameter is a value determined at the lastperformed calibration operation.
 23. The device according to claim 21,wherein said environmental parameter is selected from the groupcomprising humidity, temperature and electrical field strength.
 24. Thedevice according to claim 21, wherein determining a present value of anenvironmental parameter comprises acquiring information from a sensorintegrated in said fingerprint sensing device.
 25. The device accordingto claim 21, wherein determining a present value of an environmentalparameter comprises acquiring information from a sensor external to saidfingerprint sensing device.
 26. The device according to claim 21,wherein determining a present value of an environmental parametercomprises acquiring information via a network connection.
 27. The deviceaccording to claim 19, wherein said control unit is further configuredto initiate a second calibration operation of said fingerprint sensingdevice a predetermined time after a first operation has been performedresulting from said change in environmental parameter.
 28. The deviceaccording to claim 19, wherein said group of predetermined eventscomprises events requiring a user interaction with said user device inwhich said fingerprint sensing device is arranged.
 29. The deviceaccording to claim 19, wherein said group of predetermined eventscomprises an incoming call, an alarm, an incoming message and anincoming notification.
 30. The device according to claim 19, whereinsaid event comprises a change in a network connection.
 31. The deviceaccording to claim 30, wherein said change in network connectioncomprises a change in cell ID in a cellular network.
 32. The deviceaccording to claim 30, wherein said change in network connectioncomprises a change of strength in a wireless network connection.
 33. Thedevice according to claim 19, wherein said event comprises a change inlocation of said user device in which said fingerprint sensing device isarranged.
 34. The device according to claim 19, wherein said eventcomprises connecting a charger to said user device in which saidfingerprint sensing device is arranged.
 35. The device according toclaim 19, wherein said control unit is further configured to wait for aminimum period of time between consecutive calibration operations. 36.The method according to claim 19, wherein said control unit is furtherconfigured initiate a second calibration operation of said fingerprintsensing device a predetermined time after a first operation has beenperformed resulting from said indication of said event.